All synthetic fibers, including polyester fibers, can be classified into two groups, namely (1) continuous filaments and (2) fibers that are discontinuous, which latter are often referred to as staple fibers or cut fibers. This invention provides improvements relating to the latter group. Such polyester staple fibers have first been formed by extrusion into continuous polyester filaments, which are processed in the form of a tow of continuous polyester filaments, before the filamentary tow is converted into staple, which is then spun into spun textile yarn, often from blends of polyester fiber with other fibers, mostly cotton fibers or other natural and/or synthetic fibers.
Spinning such staple fibers (which are discontinuous) into continuous yarns or threads, which are generally referred to as "spun yarns" to distinguish them from continuous filament yarns, is one of the oldest processes known to human beings, for instance the use of a spinning wheel. Earlier in the present century, the process generally used commercially was "ring spinning". More recently, however, ring spinning is being mostly replaced by other methods, primarily "open-end spinning", sometimes referred to as "rotor spinning", and by air jet spinning. Aspects of the open-end spinning process, improvements in which are provided by the present invention, have been discussed and described in numerous publications over the last three decades, including, for example, Yngve et al. U.S. Pat. No. 4,729,214, which describes a specific improvement in a particular type of open-end spinning technique, and Ulku et al, in Textile Research Journal 65(10), 557-563 (1995), which discusses the effects of opening roller speed on various different types of fibers in open-end spinning. So far as we know, however, little has been published in the art about the effects on open-end spinning of using fibers of different cross-section.
Open-end spinning is sometimes referred to as OES herein. OES provides a different softer yarn structure than that obtained by air jet spinning. The consequently softer aesthetics of OES yarns are preferred for many end-uses, air-jet yarns having harsher aesthetics because of their different formation and their resulting different yarn structure. The pilling performance of the two yarn structures also differ.
Virtually all polyester staple fibers used to make commercial yarns for the apparel market (except for those in some selected specialty applications) have been of round cross-section for practical and economic reasons. The cross-sectional shape is established by the fiber producer primarily during melt-spinning and is then essentially fixed during drawing and annealing steps used to strengthen the fiber and to stabilize the fine structure of the polyester. Once established by the fiber producer, the cross-section of staple fiber generally remains essentially unchanged during subsequent mill processing steps used to form the yarns, fabrics and garments. Increasing the complexity of the cross-sectional shape (i.e., making and using any cross-section other than round) has generally increased processing difficulty and costs for fiber producers and especially for fiber processors.
Fiber producers prefer to manufacture round fibers over non-round fibers because melt-spinning (extruding) round filaments is most efficient and economical. Round orifices can be easily and economically fabricated. Further, melt-spinning processes used for round filaments are less demanding than for non-round filaments in that filament formation requires less strict control of polymer viscosity and air quenching to achieve acceptable quality. Immediately after extrusion, the melt tends to swell and form a bulge under the capillary orifice. Additionally, the uniform and symmetrical surface of the round shape minimizes directional influences during the filament-forming operation and maximizes the opportunity for increasing uniformity of fiber tensile, crimp and lubrication properties, uniformity generally being highly desirable.
Likewise, textile processors have preferred to process round fibers over non-round fibers in their normal processing operations because round fibers are easier and more cost-efficient to transform into spun yarns and fabric. This has been the case particularly in the textile operations of carding, drafting and spinning used to transform the raw cut polyester staple fiber into spun textile yarn. No doubt this has resulted partly from the better property uniformity as discussed above and partly from the uniform friction and processing characteristics of the symmetrical round surface.
Round fibers have also been highly desirable for their economic dyeability and coloring characteristics. Of all potential cross-sections, round fibers possess minimum surface area to color and, therefore, require less dyestuff for coloration, in contrast to any non-round cross section which must necessarily have increased surface area, so must dye with lower yield and, therefore, generally requires a higher level of costly dyestuffs to achieve the same coloration as a round cross-section.
As indicated, both fiber producers and textile mill operators have been driven by economic considerations, so polyester fibers with non-round cross-sections have found little to no use in high volume commodity polyester/cotton blend applications for the commodity apparel market. The few examples of non-round fibers in the apparel market have been limited to specialty fibers that have provided marketable visual and/or performance fabric and garment attributes that have commanded point of sales premiums to off-set the necessary added producer and textile mill costs.
This invention, in contrast, provides a commodity polyester fiber of non-round cross-section that provides, surprisingly, a combination of advantages, namely improved open-end spinning performance over round fibers as well as dye yields equivalent or near equivalent to round fibers, as will be explained hereinafter.